How to make low poly water?
Rendered in LuxCoreRender just for fancy look.

I'm not sure which cube you are referring to, so I'll first send you to some fluid sim 101 and then I'll show you what have I used for this scene.


Blender Docs
Blender fluid simulation tutorial in one minute


Now you know how to make simple Fluid sim and here is how I've achieved this low poly look:

Domain:

I've used resolution - 24. You can tweak it as needed. Lower value = lower poly count.



Inflow Object (Cube):

Here I've changed only Inlfow Velocity to -5m/s on Z axis. To speed up fluid.



Cycles:

If you want to render it with Cycles you can use this material for Domain object. But if you are gonna to use it with Unity this is optional.